(a) Technical Field
The present invention relates to a method for manufacturing palladium-platinum core-shell catalysts for fuel cells and, more particularly, palladium-platinum core-shell catalysts that contain palladium-platinum core-shell nano particles having superior durability and catalyst activity.
(b) Background Art
There is a growing global interest in next-generation energy sources because of the impending exhaustion of fossil fuel resources. For example, hydrogen fuel cells have been actively studied in academic and industrial fields as an alternative eco-friendly energy source that does not produce any pollution. In particular, hydrogen fuel cells for vehicles are expected to replace conventional combustion engines based on petroleum in the near future.
A proton exchange membrane fuel cell (PEMFC) is a system that generates power by direct electrochemical reaction of hydrogen. PEMFCs are an eco-friendly energy source because hydrogen is oxidized in an anode and oxygen is reduced in a cathode, such that power is generated and water is the by-product of the reaction: other pollutants are not generated. A PEMFC has a relatively low operating temperature of about 50-100° C. and high energy density. For this reason, the PEMFC can be used as a compact energy source for residential use, as well as an engine for a vehicle. However, PEMFCs suffer from a number of disadvantages, including: a low-power energy density (e.g., due to low reaction speed), a requirement for a substantial quantity of platinum catalysts, a requirement for the removal of vapor generated on the surface of the electrode, etc.
At present, much research has been conducted to solve the foregoing problems for commercialization of the PEMFC. In particular, research for improving cell efficiency through improvement of fuel cell catalysts has been drawing a key interest. As a fuel cell catalyst, a pure platinum catalyst for oxygen reduction is conventionally known as having the highest activity. However, as a demand for platinum has increased, so has the price of platinum, which has increased substantially over the past ten years. Due to the increase of the price of platinum, it is desirable to synthesize a catalyst having higher electrical activity than pure platinum to reduce the use of platinum. To that end, research is being actively conducted on synthesis of a core-shell catalyst.
As to conventional core-shell catalyst synthesis, one conventional strategy has proposed an electrode catalyst in which a catalyst particle including an M-core/M-shell structure having palladium as an inner particle core, palladium and platinum as an outer particle shell is supported on a support (carbon black, graphite). Unfortunately, the catalyst activity of such an electrode catalyst is relatively low because a colloidal dispersion liquid is not removed.
Another conventional strategy has proposed a method for manufacturing an electrode catalyst for a fuel cell, by which an active particle of a core-shell structure including a metal compound particle composed of palladium and platinum, or a platinum-containing alloy coating layer formed on the entire surface of the particle, dissolves a precursor compound including palladium, thus manufacturing a precursor liquid, and the metal compound particle is dried while being dip-coated in a catalyst substrate. Unfortunately, a disadvantage of this method is that the size of the catalyst particle is not uniform.
Additionally, when a palladium-platinum core-shell nano particle catalyst is synthesized using an underpotential deposition (UPD) method, interaction with the medium generated during reduction of platinum and oxygen existing on the surface becomes weak due to the influence of the core substance (e.g., palladium), such that 5 times or more of activity per unit mass is shown. It has also been shown that the palladium-platinum core-shell nano catalyst synthesized in this way has high durability as a catalyst because the core substance improves the stability of platinum of the shell. Unfortunately, the foregoing method synthesizes the catalyst on an operating electrode, a carbon electrode, such that mass production is inevitable and nano particles of a uniform size are difficult to synthesize.
Moreover, in case of the above-described palladium-platinum core-shell nano particle synthesized using the previously reported colloidal synthesis method, the particle size is as large as 20 nm or more, and nano particles in which platinum particles are reduced in the shape of a branch on the palladium core are reported. Accordingly, there is a need for a method of synthesizing a nano-particle catalyst of 10 nm or less in the palladium-platinum core-shell form.